1. Field of the Invention
The present invention is related to a letterpress printing machine. Especially the present invention suits to form a luminescent layer of a macromolecular system organic electroluminescent display unit by printing. In addition, in this case, the ink having a macromolecular system organic material dissolved in a solvent is printed on a substrate.
In addition, the present invention is related to flexographic printing method. In addition, the present invention is related to a letterpress printing machine with the use of a plastic plate. According to the current invention, minute pattern can be formed on a substrate uniformly. According to the current invention, minute pattern of high position accuracy can be formed on a substrate. According to the current invention, minute pattern can be formed on a substrate continuously and stably. More particularly, the present invention is related to an improvement of letterpress printing machine suitable for formation of the following high minute pattern: Pattern of color filter for liquid crystal displays (LCD); a luminescent layer and a charge transport layer of an organic electroluminescent element; an electrode pattern of organic thin film transistor (TFT) substrate; and a sealed pattern in electromagnetic wave shielding.
2. Description of the Related Art
In late years, as a display device in portable telephone, PDA (a personal digital assistant), a mobile PC and a navigation system adapting for car, an organic electroluminescent element comprising the following characteristic attracts attention:    1. Thin;    2. Low power consumption; and    3. A bright display.
For example, as for this organic electroluminescent element, anode (transparent conductive film, ITO film) and a luminescent layer containing an organic luminous body and a cathode (metal electrode) are formed on a transparent substrate.
As method to form minute pattern having the following characteristic on a substrate continuously and stably, a photo-lithography method is used mainly now.    1. Intra-plane uniformity; and    2. High accuracy of position.
In this photo-lithography method, a manufacturing cost is high for reasons of the follows.    1. Process is complicated; and    2. Manufacturing facility is expensive.
In addition, offset printing can be used instead of photo-lithography method. Offset printing is explained below.
A blanket comprising silicone system rubber is used. Ink transfers from a blanket to a substrate such as a glass.
Offset printing has the following defect.    1. Ink at non-printing area is removed by a press plate.    2. And this ink is disposed.
Therefore, a flexo printing method which is superior in ink mileage attracts attention instead of offset printing.
A luminescent layer of an organic electroluminescent element is usually formed by vacuum deposition of a low molecular organic luminous body. For this case, there is a limit for upsizing of an element from the viewpoint of evaporation apparatus.
Thus the following attempt is proposed.
Ink is made by dissolving a macromolecular organic luminous body in a solvent. Using this ink, a luminescent layer is formed by a well-known printing method. (e.g. Patent Reference 1 cited below).
Such a printing method can be superior to mass production characteristics. In addition, a manufacturing cost can be lowered. Offset printing or gravure printing are nominated for such a printing method.
On the other hand, flexographic printing is the printing method how flexible relief printing plate comprising rubber or resins, an anilox roll and ink are used. Conventionally, flexographic printing has been widely used in manufacturing simple printed matters such as wrapping papers. This flexographic printing is especially suitable for formation of a thin and stable print layer of about 0.01-0.2 μm in thickness. In flexographic printing, a relief printing plate part where printing pressure is applied has flexibility. In addition, in flexographic printing, printing with extremely low printing pressure called a kiss touch is performed.
Therefore, flexographic printing is suitable for printing to the following substrate.    1. A glass substrate; and    2. The substrate on which the transparent electrodes having a characteristic capable of being damaged by high pressure are formed.
Therefore, flexographic printing is a printing process especially suitable for formation of a luminescent layer of an organic electroluminescent element.
Conventional flexographic printer is explained in FIG. 4. Flexographic printer has the following member shown in FIG. 4: Anilox roll (anilox board) 101 which supplies ink to flexographic plate (relief printing plate) 105; rotary printing cylinder 102 on which flexographic plate (relief printing plate) for pattern formation 105 is loaded through cushion 104 under a plate; substrate surface plate 103 on which substrate 111 is put; and doctor blade 112 scraping off unnecessary ink on anilox roll 101.
In addition, thickness of flexographic plate (relief printing plate) 105 is measured beforehand. And distance between printing cylinder 102 and substrate surface plate 103 at the time of printing are decided. And while keeping the decided distance uniformly, ink transfers.
The relief printing plate used in flexographic printing comprises materials such as a resin or rubber which can be transformed by pressure. Therefore, in flexographic printing, high printing pressure is not required. In addition, the relief printing plate does not comprise hard material such as a metal and a glass. Therefore, even if substrate is the material which is easy to be damaged (e.g. glass substrate), it can be printed.
In the case of printing, ink is supplied to an anilox roll from an ink feeding mechanism. Subsequently a constant amount of ink transfers to a relief printing plate because a relief printing plate held by printing cylinder touches an anilox roll. Finally ink transfers from a relief printing plate to a substrate. Printing is performed in this way.
According to such flexographic printing, precise pattern can be formed on a substrate such as a glass. In addition, by relief printing using a resin as a material of a printing plate, precise pattern can be also efficiently formed on a substrate such as a glass.
In addition, in printing, pressure between a printing plate and a substrate is important. For example, in a gravure printing, ink in concave portion transfers to substrate by applying very strong pressure. On the other hand, by relief printing and flexographic printing, ink on projection part transfers to a substrate.
Therefore, in relief printing and flexographic printing, high pressure required in a gravure printing is not necessary.
However, it is necessary for a printing plate to touch substrate surely. Thus thickness of a printing plate and a cushion between a printing plate and printing cylinder is usually considered beforehand. And distance between printing cylinder and a substrate surface plate is decided. While fixing distance is determined here, printing is performed.
Case of flexographic printing is explained below.
Pressure-sensitive film is put between a printing cylinder and a substrate surface plate before printing. And a variation of printing pressure within a printing face is observed when pressure is applied. Or change of nip width by pressure application is observed when the printing plate on which ink is applied touches a substrate surface plate at a fixed point. While confirming a variation of printing pressure and nip width in this way, distance between printing cylinder and a substrate surface plate is modified. Till printing pressure or nip width within a printing face falls within a predetermined range, confirmation and adjustment are repeated. In addition, in order to absorb an impact at the time of printing and a variation of printing pressure, material of cushioning characteristics can be placed on a substrate surface plate (e.g. Patent Reference 3, Patent Reference 4).    Patent Reference 1: Japanese Patent Laid-Open No. 2001-185352 Official Gazette    Patent Reference 2: JP-T 2000-511835 Official Gazette    Patent Reference 3: Japanese Patent Laid-Open No. 8-244194 Official Gazette    Patent Reference 4: JP-T 2003-502175 Official Gazette    Patent Reference 5: Japanese Patent Laid-Open No. 2005-59348 Official Gazette(Problem 1)
However, when, for example, an organic electroluminescent element is used in full color display unit purpose, it is necessary to apply inks independently about a luminescent layer of RGB. Therefore, minute pattern of a 150-200 ppi level is formed by flexographic printing. However, while a minute pattern printing is performed consecutively, dry ink accumulates in the surface and concave portions of a relief printing plate. And defectiveness such as fluctuation of printed line width and printed film thickness occurs. Especially, drying speed of macromolecular organic luminous ink is fast because the ink uses a volatile organic solvent. Therefore, deposit of ink is easy to occur. Therefore it is necessary to wash a relief printing plate in an interval of continuous printing.
On the other hand, cleaning of a relief printing plate should be performed without taking off a relief printing plate from a printing cylinder to keep positioning accuracy of a printed pattern.
As a prior art with respect to a printer loaded with the cleaning device which a relief printing plate are washed without a relief printing plate being taken off from a printing cylinder, a technique shown in e.g. Patent Reference 2 is known.
However, such a letterpress printing machine is maintained at the time of printing end. And washing method of such a letterpress printing machine is a method rubbing against a relief printing plate surface by a brush roll. Therefore, damage to a relief printing plate is big when a relief printing plate having minute pattern is washed regularly in an interval of presswork. Therefore, good printing cannot be performed. In addition, a relief printing plate is hard to get used to ink after having done cleaning of a relief printing plate. Therefore, enough inking is unable. Therefore, defectiveness such as reduction of line width or tackiness in printed matter occurs. Therefore, till good printing is possible, waste printing to about 1-5 substrates is necessary. On the occasion of mass production, there is a problem that a large quantity of substrates is used in periodical waste printing.
The present invention solves the conventional problems such as the above mentioned problem. The present invention provides a letterpress printing machine having the following characteristic: A relief printing plate can be washed while a relief printing plate is installed in a printing cylinder; and a relief printing plate can be washed without damaging a relief printing plate.
By enhancement of ink wettability to a relief printing plate, good printing is possible immediately after washing a relief printing plate.
In addition, using this relief printing plate, a luminescent layer of a macromolecular organic electroluminescent element can be formed.
(Problem 2)
Further, the following problem occurs when printing is performed while maintaining definitely the distance between printing cylinder 102 and substrate surface plate 103. As for cushion 104 put on printing cylinder 102, flexographic plate 105 and substrate, the thickness within a printing face is uneven. Therefore, printing pressure is uneven in a printing direction and a direction that is perpendicular to the printing direction. Therefore, when fine printing is performed, this unevenness affects print quality adversely. In addition, in flexographic printing, pressure-sensitive film is put on a substrate surface plate before printing. And unevenness of printing pressure within a printing face is observed when pressure between a substrate surface plate and a printing cylinder is applied.
In addition, printing plate which is applied ink touches a substrate surface plate in a fixed point. Then nip width can be observed.
While observing printing pressure or nip width, height of a substrate surface plate is adjusted.
Adjustment by above methods is repeated till printing pressure or nip width within a printing face is fixed.
In addition, after having adjusted height of a substrate surface plate, a cushion under a printing plate and a plate is changed. Then, due to individual difference of each member or printed pattern configuration, adjustment of nip is necessary each time. Therefore, operation load is enlarged.
On the other hand, thickness of a printing plate and a material of cushioning characteristics is several hundred μm—several mm respectively. Therefore unevenness of those thickness within a printing face is more than dozens of μm. Thickness and unevenness of thickness of substrate are similar to those of a printing plate and a material of cushioning characteristics.
During printing, distance determined beforehand between printing cylinder and substrate surface plate is kept. Therefore, printing pressure varies due to unevenness of thickness of printing plate or the like.
Change of printing pressure affects a printing result. Printed matter such as a book or a packaging material can permit change of such a printing pressure. However, when printing object is precise member which requires ink of thickness from several μm to several nm, change of such a printing pressure is a fatal problem. In addition, in consideration of unevenness of thickness of printing plates or the like, printing pressure can be adjusted so that an acceptable printing result can be obtained.
However, by a replacement of a printing plate and a cushion under a plate, complicated positioning is necessary each time. Therefore, there is a problem that operation load and loss of time are enlarged.
The present invention solves a conventional problem such as the above mentioned problem. It is an object of the present invention to provide the letterpress printing machine which enables a stable high quality printing.
In letterpress printing machine of the present invention, even if thickness of a printing plate, a cushion under a plate or the like is uneven, printing pressure within a printing face between a printing plate and substrate should be kept uniform at the time of printing.
(Problem 3)
Ink transposition to substrate consisting of a glass or the like must be performed in the lower part of printing cylinder. Therefore, ink application and ink transposition cannot be performed at the same time.
At first ink is applied to a flexographic plate from an anilox roll. Afterwards printing cylinder is moved to the upper part of a surface plate. And ink transposition to substrate is performed.
As a result, the time when ink is on flexographic plate is about 10-20 seconds. When ink in a flexographic plate dries in the meantime, ink does not transfer on substrate. Therefore, drying of ink is prevented by mixing high boiling point solvent with ink.
A method of only using ink by one printing is supplied using flat anilox board is proposed as other ink application method (e.g. Patent Reference 5). However, in the case of this method, ink application and ink transposition cannot be performed at the same time. Therefore, it is necessary to use the ink which includes high boiling point solvent. However, it is necessary for a printed matter such as a luminescent layer of an organic electroluminescent element formed using ink including high boiling point solvent to be dried at high temperature. In addition, residual of a solvent is easy to occur. Besides, as for the luminescent layer of an organic electroluminescent element, a characteristic deteriorates due to heating and a residual solvent. Therefore, luminous efficiency falls. In addition, characteristic life shortens.
In addition, time for ink applied on the surface of a flexographic plate transferring on substrate was about 15 seconds when printing was performed with a conventional printer using ink including a solvent. Both ink including methyl anisole and ink including xylene dried on a flexographic plate. Therefore, these inks did not transfer on a substrate.
Thus ink which methyl anisole included 15% cyclohexylbenzene (CHB, boiling point 239 degrees Celsius) was used. For this case, ink transferred on a substrate. By a vacuum dryer, this substrate was dried at 160 degrees Celsius. But the solvent has remained without completely volatilizing. An organic electroluminescent element was further made using this substrate. However, it was confirmed that luminous efficiency largely deteriorated. For example, the luminous efficiency when an organic electroluminescent element of an active method was driven at drive voltage 5V was about 6.8 cd/A.
The present invention solves a problem such as the above. The present invention provides a letterpress printing machine having the following characteristic: The machine can produce an inexpensive printed matter such as a macromolecule organic electroluminescent element of high luminous efficiency and long characteristic life.